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Forum: Analog Circuits Operational Amplifier Power Problem


Author: Kynix L. (kynix_l)
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I want to use the op amp as a voltage follower. The input voltage range 
is between 1V and 45V. I have found a few high-voltage power amplifiers 
on the Internet. 2142 seems to be fine. However, the datasheet says that 
it can be powered by a single power supply or dual power supply. VCC+ is 
+22V and VCC- is -22V. Then if I want a single power supply, add +44V to 
the positive power supply, can the negative power supply be grounded? If 
not, is there any other way?
I ask that the output voltage does not have to be very accurate. It is 
required to output 1V~45V, and the actual output can be about 1V~40V. In 
this way, I put the voltage of 45V to the operational amplifier as the 
positive power supply and the negative power supply to the ground. At 
the same time, it is also connected to the 45V of the same input 
terminal. Should it output about 43V?
I looked through many passages about it on the Internet, like 
Operational Amplifier Principle and 
Circuit(http://www.apogeeweb.net/article/163.html) and so on, but I just 
confused by all kinds of articles.
So if you have any good ideas, could you please share them to me?

: Moved by Moderator
Author: Dieter (Guest)
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Single supply: Look for virtual ground  U/2.

Author: Lothar M. (lkmiller) (Moderator)
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Kynix L. wrote:
> I put the voltage of 45V to the operational amplifier as the positive
> power supply
With 45V as suplly you will not reach the 45V at the output as the 2142 
high voltage amplifiers are no rail-to-rail ampilfiers.
Also you will have a problem with the common mode input voltage range 
then:  the input range limited to Vpos-2V.
Additionally the 44V is an absolute maximum value. You should stay a 
good portion away from that to ensure long life of your design.

Lets look at the OPA454: the common mode input voltage range is
(V–) + 2.5V    to   (V+) – 2.5V
So you will need a negative supply of -1.5V and a positive supply of at 
least 46.5V to satisfy the input stage.
And the output swing is defined as (V–) + 2V  and (V+) – 3V, so you will 
need a supply of -1V and 48V for the desired output range from 1..45V.
All in all this amplifier therefore will need -1.5V and 48V as suplly to 
fit your needs.

The LTC6090 is called a rail-to-rail device, but a closer look shows 
that this is only valid for the output. The inputs mus be in the range 
of V
V– + 3V to V+ -3V resulting in a supply of -2V and +48V for a simple 
follower.

Now you see, that selection is not only looking for the supply voltage, 
but also for the input range and the output swing.

Author: Peter (Guest)
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Kynix L. wrote:
> 2142 seems to be fine.

What means "2142"?

For 45V Input I would suggest an opamp, which was rated to at least 50V.
If you want to omit a negative power supply, search for an opamp, which 
accept input voltages including the negative rail.
On the other side you need typical to be about 3V below the positive 
rail. So the power supply should deliver at least +48V.

Author: Peter (Guest)
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E.g. the ADA4522 seems applicable.

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